1. Field of the Invention
This invention relates to a surface-acoustic-wave parametric device for use as a variable-frequency selecting device. More particularly, this invention relates to a surface-acoustic-wave parametric device wherein a plurality of pumping electrodes are arranged in the direction of propagation of surface acoustic waves, and values of DC bias voltages or pumping voltages for forming parametric interaction regions, which are to be applied to the respective pumping electrodes, are differentiated from each other so as to correspond to a desired output frequency characteristic, thereby enabling desired design of a frequency characteristic of the variable-frequency selecting device.
2. Description of the Prior Art
One of the inventors of the present invention has already disclosed, in Japanese Laying-Open No. 54-41089 (1979), a surface-acoustic-wave device having a variable frequency selecting function as illustrated in FIG. 1.
In FIG. 1, numeral 1 designates a semiconductor substrate, and an insulator film 2 and a piezoelectric layer 3 are laminated on the semiconductor substrate 1. A rectangular pumping electrode 4 to which a DC bias voltage and a pumping voltage are applied and input and output transducers 5 and 6 are arranged on the piezoelectric layer 3.
Numeral 7 designates a DC power source for applying a DC bias voltage, 8 designates an inductor for AC blocking, 9 designates a high-frequency power source for applying a pumping voltage, 10 is a capacitor for DC blocking, and 11 and 12 designate surface-acoustic-wave absorbing members for preventing undesired reflection of surface acoustic waves at the ends of the device.
The DC bias voltage is applied from the DC power source 7 to the pumping electrode 4 so as to create a suitable depletion-layer capacitance at a surface portion of the semiconductor substrate 1 under the pumping electrode 4. Further, the pumping voltage having a frequency which is twice that of a center frequency fo of a desired frequency band is applied from the high-frequency power source 9 to the pumping electrode 4 so that the depletion layer capacitance is oscillated and modulated at the frequency 2fo.
When an electric signal is applied to the broad-band input transducer 5, the input electric signal is converted into a surface-acoustic-wave signal which is propagated on the surface of the piezoelectric layer 3 rightwardly and leftwardly as viewed in FIG. 1.
When a signal component of the surface-acoustic-wave input signal 13 propagating in the rightward direction and having a frequency around fo passes through an operating region under the pumping electrode 4, the piezoelectric potential thereof is subjected to a parametric interaction with the pumping voltage due to the depletion layer capacitance non-linearity effect on the surface of the semiconductor substrate 1 so that the component is amplified. This amplified surface-acoustic-wave signal 14 is converted into and outputted in the form of an electric signal by the output transducer 6.
At the same time, a surface-acoustic-wave signal 15, which has a frequency fi (fi=2fo-fs, fs: a frequency of the input signal) corresponding to the amplitude of the surface-acoustic-wave input signal 13, is also produced from the pumping electrode 4 and propagated leftwardly as viewed in FIG. 1. This surface-acoustic-wave signal 15 may also be outputted as an output signal.
The frequency characteristics 14a, 15a, 14b and 15b of the respective output surface-acoustic-wave signals 14 and 15 are shown, in FIGS. 2 and 3, in relation with the input signal 13 whose amplitude is shown as 1 in the figures. FIG. 2 shows the case where the pumping voltage is relatively small and FIG. 3 shows the case where the pumping voltage is relatively large.
As apparent from FIGS. 2 and 3, in the surface-acoustic-wave device having a rectangular pumping electrode, a response at a signal passing band and a spurious response are substantially determined when an output at a desired center frequency fo is selected. By this reason, when the conventional surface-acoustic-wave device is used as a frequency selecting device, the frequency characteristic cannot be designed freely. And yet, the spurious response is still too high to be practically used.